10.2 Enzymes Required for the Degradation of Lignocellulosic Waste

145

Table 10.1

Types of substrates used by fungi for cellulase production.

Fungi

Substrates

Aspergillus niger

Corn cob, Sorghum straw

Alternaria alternata

Corn cob

Aspergillus oryzae

Soybean hulls

Trichoderma reesei

Soybean hulls

Aspergillus japonicas URM5620

Castor bean

Thermostable yeast

Bagasse powder

Aspergillus terreus

Rice straw

Source: Modified from Gunjal et al. [1].

Some bacteria can produce cellulase even in extreme conditions. For example,

Pseudoalteromonas haloplanktis found in sea water of Antarctica produces a

cellulase, Cel5G which is psychrophilic in nature. Thermophilic bacteria like

Clostridium sp., Fervidobacterium sp., R. marinus, Geobacillus sp., Acidothermus

cellulolyticus, Thermotoga sp., Caldicellulosiruptor sp., and Anaerocellum ther-

mophilum also secrete thermostable cellulase which helps in the degradation of

lignocellulosic wastes [1].

10.2.1.2

Enzymes Responsible for Cellulose Degradation

The main enzyme required for cellulose breakdown is cellulase. The enzyme has

both catalytic and non-catalytic cellulose binding modules (CBMs). The CBM helps

in binding to the surface of the cellulose, and the catalytic domain of the enzymes

facilitates the breakdown of the β-1,4-glycosidic bonds of cellulose producing glu-

cose units [17]. The end products of cellulose degradation are CO2 and water (aerobic

condition), and CO2, water, and methane (anaerobic condition) [18].

Three types of enzymes are used in cellulase degradation viz., β-1,4-endoglucanases

(EGL) or Endoglucanases, exoglucanases/cellobiohydrolases (CBH), and Cellobiase

or β-glucosidase (BGL). The EGL acts in the amorphous region and breaks internal

bonds releasing the cellulose chains (endo-cleaving). The CBH acts at the terminal

part of the polysaccharide chain and releases cellobiose units (exo-cleaving) [19].

Finally, BGL breaks down cellobiose and/or remaining oligosaccharide chains into

glucose units, which becomes the carbon source for the microorganisms [20]. All

the three enzymes, i.e. EGL, CBH, and BGL, must act in a synergistic and sequential

way for the complete degradation of cellulose.

10.2.1.3

Physical Pre-treatments to Break down Cellulose

Apart from the biological methods used for the breakdown of lignocellulosic wastes,

there are also some physical methods that are adopted by many industries. The

pre-treatments’ aim will facilitate the breakdown of cellulose by increasing the

surface area of the wastes. Surface area is increased by reduction in the particle size,

crystallinity, and degree of polymerization. Reduction in the particle size is achieved